This is really hard to say. These factors can be pinpointed by a couple of things.
Firstly, optimize your website as fully as possible. This would include caching,gzip compression, optimizing images etc. etc.
Secondly, what setup do you want to use for your server? Is it Apache with Varnish,Nginx with Varnish, plain Nginx or Varnish with Pound?
For me the best possible configuration for a Wordpress website is Nginx however maybe someone else can also chipin and say what they prefer.
Another thing would be using MariaDB 10.3(I think that was the latest at the moment) for a DB engine. I find that it handles Wordpress and the queries it generates quite well.
With all that combined, I’ll recommend starting with a droplet of the size of your VPN and gradually upgrade it, if needed.
You can also try and separate the DB server for the Web Server. That way the traffic will be load balanced a bit more.